Diffractive device for generating one or more diffracting images including a surface relief structure at least partly arranged in a series of tracks

ABSTRACT

A diffractive device having a surface relief structure which, when illuminated by a light source, generates one or more diffraction images which are observable from particular ranges of viewing angles around the device, wherein at least part of the surface relief structure (1) is arranged in a series of tracks (2), each track (4, 5) having a diffracting surface (3) which generates a component of a diffraction image, such that at least one of the diffraction images generated by the diffractive device is formed from image components generated by a plurality of the tracks.

This invention relates to a diffractive device. It relates particularlyto a diffractive device which, when illuminated by a light source,generates one or more diffraction images which are observable fromparticular ranges of viewing angles around the device. The device may beused in a number of different applications, and it has particularapplicability as an anti-forgery security device on banknotes, creditcards, cheques, share certificates and other similar documents.

Several different types of diffractive devices which, when illuminated,generate diffractive images, are known. In January 1988, an Australianten dollar banknote was released featuring a diffractive image ofCaptain Cook. The diffractive grating used in the image was for the mostpart comprised of substantially continuous lines, and the shapes andconfigurations of the lines were determined according to opticalcatastrophe theory in order to generate fine detail in the diffractiveimage observed.

International patent application PCT/AU90/00395, the contents of whichare incorporated herein by reference, discloses an alternative methodfor generating an optical diffraction image. In this case, thediffractive device is divided into a large number of small diffractiongrating structures, each of which diffracts a beam of light which actsas a pixel, with the pixels combining to form an overall image.According to preferred aspects of the arrangement disclosed, therespective diffraction grating of each pixel comprises a plurality ofreflective or transmissive grooves or lines which are usually curvedacross the pixel. Groove or line curvature determines both local imageintensity (eg. shading) and local optical structure stability. Groove orline spacing in each pixel grating determines local colour properties,with non-primary colours generated by a pixel mixing. Average groove orline orientation determines movement or colour effects. The overallsurface structure of each pixel grating is selected from a palette ofdifferent grating types having a limited number of distinct values ofaverage curvature and average spacing.

An advantage of the use of pixel gratings in a diffractive device isthat it permits the device to generate more than one diffraction image.Some of the gratings can have diffractive surfaces with particular linespacing curvature and orientation characteristics which contribute tothe generation of an image viewable from a particular range of viewingangles, and other gratings have different surface characteristicscontributing to the generation of a different image viewable from adifferent range of viewing angles. This result is much more difficult toachieve in a continuous grating diffractive device.

Another advantage of a pixel grating diffractive device is that itallows storage of picture information in a digital format. However, apredetermined surface area on the diffractive device must be set asidefor each pixel, and this is not the most efficient way of storingpicture information in a limited space. Accordingly, there is scope fora more efficient manner of storing picture information in a diffractiongrating.

Moreover, in a pixel grating diffractive device, there are inevitablediscontinuities between adjacent gratings. Diffraction effects occur inthese discontinuities. It is normally possible to ensure theseextraneous diffraction effects are small relative to the intentionaldiffraction effects generated by the diffractive device, but theextraneous diffraction effects are still detectable. It is desirable toreduce the extraneous diffraction effects.

According to the present invention, there is provided a diffractivedevice having a surface relief structure which, when illuminated by alight source, generates one or more diffraction images which areobservable from particular ranges of viewing angles around the device,wherein at least part of the surface relief structure is arranged in aseries of tracks, each track having a diffracting surface whichgenerates a component of a diffraction image, such that at least one ofthe diffraction images generated by the diffractive device is formedfrom image components generated by a plurality of the tracks and whereinat least some tracks have diffracting grooves or other shapes on theirsurfaces, varying continuously in terms of orientation, curvature and/orspacing along the track, the variations in orientation, curvature and/orspacing being a means by which image information is encoded into thetracks.

Tracks may be of any suitable shape, size and configuration. It ispreferred that individual tracks have a length greater than 0.5 mm. Itis further preferred that each track has a width of less than 0.25 mm. Awidth of 0.25 mm represents approximately the limit of resolution of thehuman eye when viewing a diffractive device from close quarters, so thata track having a width of less than 0.25 mm is unlikely to be separatelydiscernible to the human eye.

The tracks may be in any suitable configuration. In one preferredarrangement, the tracks are straight and parallel, in side-by-sideconfiguration. In an alternative arrangement, the tracks may form arcsof concentric circles In other arrangements, the tracks may be in theshape of curving lines.

All of the tracks may generate a component of the same diffractionimage, but it is preferred that the tracks be used to generate two ormore different images. In one arrangement in which two diffractionimages are generated, every second track contributes to one image andevery other track contributes to the other image. It is not essentialthat all tracks be of the same width, but that is a preferred feature.It is not essential that the tracks for the two images be arrangedalternately; they may occur in any order. There may be more than twotypes of tracks, associated with more than two images.

In one preferred arrangement, the diffracting surface of each trackcomprises a series of lines or grooves which extend across the width ofthe track. As an alternative to lines or grooves, it is possible to usecircles, polygons and other shapes which are capable of providing therequired diffraction effects. In another preferred arrangement, thediffracting surface comprises a pattern of parallelogram-shapedindentations.

In another preferred arrangement, the diffracting surface of each trackcomprises a series of lines or grooves which extend in a generallylengthwise direction along the track. Such lines or grooves may bestraight or curved, and in one arrangement they may be undulatingperiodically in a sinusoidal configuration. The lines or grooves may beshort and discrete, or they may be substantially continuous throughoutthe length of the track.

In an especially preferred arrangement, the surface relief structure mayinclude tracks having crosswise grooves or parallelogram patternsinterspersed with tracks having lengthwise grooves or parallelogrampatterns, such that diffraction effects from one set of tracks areobservable when the diffractive device is viewed in the direction of thetracks, and diffraction effects from another set of tracks areobservable when the diffractive device is viewed perpendicular to thedirection of the tracks.

As an optional refinement, one of the images generated by thediffracting tracks may be a uniform or blank image which can be encodedwith image information by the physical destruction or modification ofregions of diffracting surface on selected tracks to producecorresponding diffusely reflecting regions.

The invention will hereinafter be described in greater detail byreference to the attached drawings which show an example form of theinvention. It is to be understood that the particularity of the drawingsdoes not supersede the generality of the preceding description of theinvention.

FIG. 1 is a schematic representation of a region of a surface reliefstructure on a diffractive device according to one embodiment of thepresent invention.

FIG. 2 is a schematic representation of parts of the surface reliefstructure of FIG. 1.

FIG. 3 is a schematic representation of other parts of the surfacerelief structure of FIG. 1.

FIG. 4 is a more detailed schematic representation of two parts oftracks used in a diffractive device according to an embodiment of thepresent invention.

FIG. 5 is a detailed schematic representation of a part of two adjacenttracks in an alternative embodiment of the invention.

FIG. 6 shows a schematic representation of a part of a track accordingto another embodiment of the invention.

FIG. 7 shows a schematic representation of a part of two adjacent tracksaccording to an embodiment of the invention.

FIG. 8 shows a computer-generated detailed representation of a sectionof two adjacent tracks according to an embodiment of the type shown inFIG. 4.

FIG. 9 shows a computer-generated detailed representation of a region ofsurface relief diffractive structure showing several tracks according toan embodiment of the type shown in FIG. 5.

FIG. 10 is a computer-generated detailed representation of a part of twoadjacent tracks according to another embodiment of the invention.

FIG. 11 is a computer-generated detailed representation of part of twoadjacent tracks according to another embodiment of the invention.

Referring firstly to FIG. 1, part 1 of the surface relief structure isarranged in a series of tracks 2, each track having a diffractingsurface 3 which generates a component of a diffraction image. In theembodiment illustrated, two separate images are generated, one by lefthand side tracks 4, and one by right hand side tracks 5. The twodiffraction images are formed from image components generated byindividual tracks 4 and individual tracks 5 respectively.

Each of tracks 2 may be of any suitable length. It is preferred thateach track be greater than 0.5 mm in length, and for the sake ofconvenience, it is preferred that each track extend throughout thelength of the diffractive device, although there is no requirement thatthis be the case. In the preferred embodiment illustrated, each oftracks 2 is straight and arranged in parallel side-by-sideconfiguration. In alternative embodiments, the tracks may be arranged inconcentric circles or sections of concentric circles, or in many othercurved arrangements.

Each of tracks 2 may be of any suitable width. It is preferred that thetracks be sufficiently narrow to be not noticeable to the naked humaneye. The limit of resolution of a normal human eye examining adiffractive device at close quarters is about 0.25 mm. Accordingly,tracks having a width of less than this amount are unlikely to beseparately discernible to the human eye.

As stated previously, discontinuities around the borders of individualpixels in pixellated diffracting devices result in incidentaldiffractive effects. The extent of such incidental effects is diminishedby the use of tracks according to the present invention in thatdiscontinuities along the length of the track can be avoided, althoughdiscontinuities are still present along the sides of each track.

It is preferred although not essential that each of tracks 2 be of thesame width. If each track has the same width, the encoding ofdiffraction image data in the diffracting surface of each track is asimpler operation. However, in situations where it is desired that thediffractive device generate multiple diffraction images, it may bedesired that one such diffraction image be brighter than another, andone way of achieving such an effect is to devote wider tracks to thegeneration of the bright image and narrower tracks to the generation ofthe dull image.

In the embodiment illustrated in FIG. 1, tracks 2 are arrangedsubstantially in side-by-side configuration. However, it is notessential that each track abut the next track, and a channel of anydesired width may be left between adjacent tracks. It is sometimesadvantageous to leave a small channel of about 4 micron in width betweenadjoining tracks to act as an air ventilation route during production ofthe diffractive device. Diffractive devices of the type herein describedare typically manufactured by an embossing process, and it has beenfound that more satisfactory results are achieved if air ventilation canoccur.

The diffracting surface on each of tracks 2 may have any suitablediffractive surface relief structure. In the embodiment illustrated inFIGS. 1 to 3, the surface relief structure comprises a series of curvedor straight lines or grooves which extend across the width of the track.It is not essential that lines be used, and other suitable diffractiveshapes include circles and polygons. In one suitable arrangement, thesurface relief structure of a track may consist of variably shapedpolygon structures having dimensions less than 1 micron positioned alongand across each track in such a way as to encode the diffraction imageinformation and diffractively regenerate it. In another embodiment, thesurface relief structure of a track may consist of numerous diffractingdots of sizes less than 0.25 micron, such that the diffraction imageinformation is encoded in the spacing and distribution of the dots.

FIG. 4 illustrates in more detail portions of two tracks, eachconsisting of a complex generalized diffraction grating structure havinggrooves which vary continuously in terms of spacing, orientation andcurvature along the length of the track. The variations in groovespacing, curvature and orientation are the means by which thediffraction image information is encoded in the tracks. In preferredarrangements, the variations in groove spacing, angle and curvature canbe described by mathematical functions of two variables whose Hessian ofsecond derivatives with respect to the two variables is non-vanishingexcept along certain characteristic lines within each diffracting track.

One particular example of a suitable track grating function is given bythe following expression: ##EQU1## where:

Z is the track groove index parameter;

α=α(Y) along the length of the track;

β=β(Y) along the length of the track;

α is a preset variable which determines the local carrier wave frequencyof the track and therefore determines the local line density of thetrack and the colour of the image component generated by the track.Typically, 0.8<α<1.2;

β is a parameter which is set proportional to the local intensity of thecolour of the track and determines the structural stability of thetrack. It is this parameter that is used to tune the imagecharacteristics of the diffractive device. Typically, 0≦β≦0.056;

the number ranges of the local co-ordinates X and Y is given by 0≦X≦0.2and 0.2≦Y≦0.6 for a left hand channel track, and 0.6≦X≦0.8 and 0.2≦Y≦0.6for a right hand channel track; and

the Hessian of the track grating is non-vanishing except along certaincharacteristic lines of the grating plane which, under gradienttransformations, map to lines of singularity (caustics) in diffractionspace. The Hessian, H(X,Y) of Z(X,Y) is a standard complex derivativegiven by: ##EQU2##

FIG. 4 shows two track segments having track grating functions of thetype described in Equation (1) above. A single track may be comprised ofseveral such segments linked end to end, each segment being of fixed orvariable length. In arrangements where each track segment is of fixedlength, it is preferred that each segment form a "period" in a "carrierwave" encoded into the track, with diffraction image information beingencoded into each period by means of variation in groove spacing andcurvature. The track segments illustrated in FIG. 4 have a width ofabout 15 micron and a length of about 30 micron, although they can bescaled up or down in size as required.

FIG. 8 is a computer-generated representation of a section of a pair ofadjacent tracks, labelled 14 (left hand track) 15 (right handtrack)channel. The track sections illustrated form part of a largerstructure containing several left hand tracks interspersed betweenseveral right hand tracks. The left hand tracks, when illuminated,generate one or more diffraction images observable from particularpositions around the diffractive device, and the right hand tracksgenerate images observable from different positions. The track portionsillustrated are each about 15 micron in width and 60 micron in length.

As will be seen from close examination of FIG. 8, each curved grooveextending across the track is for the sake of convenience composed ofeight segments 18, each of which is a parallelogram in shape. Eachparallelogram indentation 18 is approximately two microns wide. Althoughmost parallelograms 18 match up with neighbouring parallelograms to formcurved grooves extending across the track, some add density toparticular parts of the track surface without joining up with anyneighbours.

The concept of dividing each groove into eight parallelograms 18 istaken a step further in the embodiment shown in FIG. 10. In thisembodiment, the track surface is comprised entirely ofparallelogram-shaped indentations. The dark portions represent troughs,whereas the light portions represent crests. Some parallelograms matchup with their neighbours to form grooves, but this is incidental ratherthan intentional as in the embodiment of FIG. 8. In any line across oneof the tracks in the embodiment of FIG. 10, all parallelograms have thesame angular orientation; whereas such orientation varies considerablyin the embodiment of FIG. 8.

The patterns shown in both FIG. 8 and FIG. 10 are used to generatepixels in the image planes. Each of the left-hand 14 and right-handtracks 15 in each case includes two segments (16,17), the top half 17being one segment and the bottom half 16 being another. Each segmentgenerates one pixel. The patterns shown are used to generate pixelshaving one of sixteen different greyscale values. Segments with flatterlines produce darker pixels in the image plane, and segments withsteeper lines (more sharply angled parallelograms) produce lighterpixels. A large number of track segments from different tracks can thusbe used to generate a complete image with sixteen greyscales.

In addition to the 16 different types of greyscale segments, the"palette" of different track segment types in a preferred arrangementincludes 10 different colour effects segments. The left hand track 14 inFIG. 11 contains two colour effects segments (16,17). In the embodimentillustrated, colour effects segments are created using straight grooveswhich cross the track at right angles, with varying spatial frequencies.The right hand track 15 in FIG. 11 contains two more colour effectssegments, but with grooves aligned with the track to create "90°effects"--that is, diffractive effects which are visible at positions90° around from where the left hand track diffractive effects arevisible.

An especially desirable type of colour effect is obtained when thecolours appear to move along a path in the image plane when thediffractive device is tilted about an axis in its plane. Such effectscan be obtained by sequential positioning of colour effects tracksegment types, with average spatial frequency increasing or decreasingalong the sequence.

It is preferred that the colour effects track segments be modulated sothat image components generated by those segments are observable overbroader ranges of angles than they would have been if their diffractingsurfaces were immodulated. A suitable general modulation function isgiven by: ##EQU3##

where β is a modulation factor; a is the average diffraction structurespacing; Q is the number of cycles of modulation; N is the total numberof grooves or equivalent diffraction structures within the tracksegment; m is the groove index parameter (m=1-N); and F is sin or cos oranother harmonic or quadratic function.

The spatial frequency of the vertical grooves of the right hand track inFIG. 11 is the same at the top and bottom of each segment, and changesthrough several steps to a characteristic frequency in the centre 19 ofeach segment.

The right hand track 15 in FIG. 10 has a different average spacialfrequency from the left hand track 14 in order to reduce the likelihoodof interference between the two different images which are to begenerated. Moreover, the parallelograms 18 in the left and right trackshave opposing angular orientations.

Track surface patterns of the types illustrated in FIGS. 8, 10 and 11are typically created using an electron beam. A 30 micron by 30 micronsurface area is typically divided into a grid of 1024 by 1024 units.This grid is then used to define the start and end points of eachparallelogram. In the embodiments shown in FIGS. 8, 10 and 11, one gridarea covers one track segment (30 micron long) in each of two adjacenttracks (15 micron wide each). An algorithm, written in BASIC programminglanguage, for generating the left hand track in FIG. 10 is given by:

J1M&=JOM&+INT((45-3*(JJ-11))*ABS(SIN(1.5708*LLL/512))

*ABS(256-XINC)/1024) 1.5

J1P&=JOP&+INT((45-3*(JJ-11))*ABS(SIN(1.5708*LLL/512))

*ABS(256-XINC)/1024) 1.5

where:

JOP is the top left corner of a parallelogram

JOM is the bottom left corner

J1P is the top right corner

J1M is the bottom right corner

JJ is the number representing the type of greyscale element (JJ isbetween 11 and 26, giving 16 different types)

XINC=64 (i.e. the width of the parallelogram, in grid positions)

LLL is a vertical index.

A similar algorithm applies for the right hand track in FIG. 10.

The diffracting tracks illustrated in FIGS. 8, 10 and 11 containdigitally encoded image information. That is, tracks are divided intosegments of a predetermined size, and a portion of image information(usually corresponding with a single pixel in the image plane) is storedin each segment. It is not however necessary that tracks be divided intoregular segments. Instead, the diffractive surfaces may varycontinuously but irregularly in terms of diffractive structure spacing,curvature and orientation, so that image information can be stored in ananalogue format rather than a digital format. In such an arrangement,the image in the image plane may be comprised of a group of lines (eachline corresponding to a track) rather than a group of discrete pixels(each pixel corresponding to one or more track segments).

One or more of the diffracting tracks may contain diffusely reflectingregions (consisting of randomly spaced grooves) and/or specularlyreflecting regions in between diffracting regions. Diffusely reflectingregions may be used to encode auxiliary information not found in thediffraction image. Specularly reflecting regions may be used to enhancethe contrast properties of the diffracted image.

One or more diffraction images which are generated by the diffractingtracks may consist of abstract colour patterns which create variablecolour effects which move along the tracks when the device is movedrelative to the light source and the observer. In particular, themovement effect may be generated when the device is rotated about anaxis in its own plane.

It is preferred that the diffracting tracks generate two or morediffraction images which are observable from different ranges of viewingangles around the diffractive device, with some of the diffractingtracks being devoted to producing each of the diffraction images. In theembodiment illustrated in FIGS. 1, 2 and 3, left hand tracks 4 aredevoted to generating a first diffraction image which is observable froma first range of viewing angles around the diffractive device, and righthand side tracks 5 are devoted to generating a second diffraction imagewhich is observable from a second range of viewing angles around thediffractive device. As illustrated in FIG. 1, the tracks are in analternating right-left-right-left configuration; however, this is notnecessary and the tracks may be arranged in any order, such asright-right-left-right-left-left.

FIG. 5 shows sections of two tracks according to another embodiment ofthe invention. Left hand track 6 has grooves extending across the widthof the track, generating diffractive images which can be observed from adirection generally along the length of the track. Right hand track 7consists of a plurality of island regions 8 surrounded by flat regions9. Island regions 8 have grooves extending lengthwise along the track,generating diffractive images which can be observed from a directiongenerally perpendicular to the length of the track. A particularadvantage of the arrangement illustrated in FIG. 5 is that diffractionimages are generated both in the direction of the length of the tracksand in the perpendicular direction, so that the diffractive effects ofthe diffractive device are more readily observable.

Flat regions 9 are optional, but they provide certain advantages. Aspreviously indicated, diffractive devices of the type described aretypically created using an embossing process, and flat regions 9 act asvents for gas removal during the embossing process, resulting in a moreprecise finished product. Moreover, an electroplating process typicallyfollows the embossing process, and flat regions 9 enable more accurateelectroplating. Flat regions 9 may also carry printed lines which areresponsive to the scan rates of particular colour photocopiers so thatmoire interference lines are created on a photocopies image of thediffractive device. Alternatively or additionally, flat regions 9 may beembossed or printed with micro-writing 13 having a size in the order of2 micron as shown in FIG. 9. Such micro-writing may serve as anadditional security element and may include a registration number orother identifier unique to the diffractive device on which it appears,thereby enabling verification of authenticity by means of microscopicexamination.

Hand track 6, islands 8 and flat regions 9 may be of any suitabledimensions. In an especially preferred arrangement, hand track 6 andisland regions 8 are each about 15 micron in width, and flat regions 9are about 4 micron in width.

In a variation on the arrangement shown in FIG. 5, each island 8 may beconnected to its neighbouring islands by means of interconnectinggrooves which may be branched, so that grooves are substantiallycontinuous throughout the length of the track.

FIG. 6 shows a track 10 having grooves which extend substantially alongthe length of the track rather than substantially across the track as isthe case in the track segments of FIG. 4. The diffraction effectsgenerated by track 10 are substantially at right angles to thosegenerated by a track comprised of track segments of the type shown inFIG. 4. Track 10 essentially comprises "carrier waves", with imageinformation being encoded into them by means of amplitude and groovespacing variations.

In some embodiments, the variations in groove spacing, angle andcurvature can be described by mathematical functions of two variableswhose Hessian of second derivatives with respect to the two variables isnon-vanishing except along certain characteristic lines within eachdiffracting track, as previously discussed. However, this is not anessential condition, and in other embodiments the Hessian of secondderivatives of the grating function may be identically zero for allpoints within the track.

FIG. 7 illustrates schematically a combination of left and right tracks,11 and 12 respectively. Left track 11 may be any one of the types oftracks illustrated in FIGS. 1, 2, 3, 4 and 8 and right track 12 is atrack of the type shown in FIG. 6. Several such left and right tracksmay combine to form a two-channel diffractive device. Tracks 11 and 12may be of any suitable width as previously discussed, and an especiallypreferred width is around 15 micron. The arrangement illustrated in FIG.7 is particularly advantageous because the image(s) produced by lefttracks 11 will be observable from angles approximately 90° around fromwhere the image(s) generated by right tracks 12 are observable.

In one embodiment of the invention, one or more of the images generatedby the diffractive device may consist of a uniform or blank image planewhich can be encoded with image information by the destruction ormodification of diffracting elements at selected locations alongselected diffraction tracks. This enables post-production modificationof the diffracting device to incorporate a new diffraction image,although the resolution of the image information so incorporated islower than the resolution normally provided by a diffracting track. Aparticular embodiment of this feature comprises a series of tracks.Along the length of each track, the diffracting surface alternatesbetween surface portions which give rise to black image components inthe image plane and surface portions which give rise to white imagecomponents. In order to create a dark area in the image plane, the"white" parts of the corresponding diffracting surface portions areerased; whereas the "black" surface portions are erased to create abright area. In this way it is possible to encode a black-and-white bitimage into the tracks.

As a further enhancement, the diffracting surfaces on some of the tracksmay include diffusely reflecting regions. Such regions do not affect theimages observed in the image phase, but they give a neutral backgroundappearance to the diffractive device, making the images more easilyobservable.

As another enhancement, some of the tracks may include specularlyreflecting regions. Such regions are useful in adding contrast to theimages observed in the image planes.

It is to be understood that various alterations, additions and/ormodifications may be incorporated into the parts previously describedwithout departing from the ambit of the present invention.

I claim:
 1. A diffractive device having a surface relief structurewhich, when illuminated by a light source, generates one or morediffraction images which are observable from particular ranges ofviewing angles around the device, wherein at least part of the surfacerelief structure is arranged in a series of discrete tracks, each trackhaving a diffracting surface which generates a component of adiffraction image, such that at least one of the diffraction imagesgenerated by the diffractive device is formed from image componentsgenerated by a plurality of the tracks, and wherein at least some trackshave diffracting grooves, or circular or polygonal, indentations orprotrusions on their surfaces, varying continuously in terms oforientation, curvature and/or spacing along the track, the variations inorientation, curvature and/or spacing being a means by which imageinformation is encoded into the tracks.
 2. A diffractive deviceaccording to claim 1 wherein each track has a width of less than 0.25 mmand at least some tracks have a length greater than 0.5 mm.
 3. Adiffractive device according to claim 1 or claim 2 wherein the tracksare straight and parallel.
 4. A diffractive device according to claim 1or claim 2 wherein the tracks form areas of concentric circles.
 5. Adiffractive device according to claim 1 or claim 2 wherein the tracksare in the shape of curving lines.
 6. A diffractive device according toclaim 1 wherein a region of the surface relief structure generates twodifferent diffraction images observable from different ranges of viewingangles, and a first group of tracks on the region generate one of thediffraction images, and a second group of tracks interspersed with thefirst group generate the other diffraction image.
 7. A diffractivedevice according to claim 1 wherein on some tracks the diffractingsurface comprises a series of grooves oriented generally across thetrack and on some tracks the diffracting surface comprises a series ofgrooves oriented generally along the track.
 8. A diffractive deviceaccording to claim 1 which includes tracks which have grooves undulatingperiodically generally lengthwise of the tracks.
 9. A diffractive deviceaccording to claim 1 which includes tracks whose diffracting surfacecomprises islands which have grooves extending generally lengthwise ofthe track, the islands being surrounded by flat regions.
 10. Adiffractive device according to claim 9 wherein the flat regions areembossed or printed with microwriting.
 11. A diffractive deviceaccording to claim 1 wherein an image generated by the device is auniform or blank image which can be encoded with image information bythe physical destruction or modification of regions of diffractingsurface on selected tracks to produce corresponding diffusely reflectingregions.
 12. A diffractive device according to claim 1 wherein on sometracks the diffracting surface comprises a pattern of parallelograms ofvarying angular orientations, indented into the track surface.
 13. Adiffractive device according to claim 1 wherein parts of the diffractingsurfaces on some tracks are assigned to generating greyscale imageinformation in the image plane.
 14. A diffractive device according toclaim 1 wherein parts of the diffracting surfaces on some tracks areassigned to generating colour effects in the image plane.
 15. Adiffractive device according to claim 14 wherein the colour effectsappear to move along a path in the image plane when the device is tiltedabout an axis in its own plane.
 16. A diffractive device according toclaim 1 wherein image information from an image generated by the deviceis encoded in an analogue manner along the length of some tracks, eachtrack generating a line of the image, the lines generated by thosetracks combining to form the image.
 17. A diffractive device accordingto claim 1 wherein image information from an image generated by thedevice is encoded in a digital manner along the length of some tracks,each track generating a line of the image, the lines generated by thosetracks combining to form the image.
 18. A diffractive device accordingto claim 1 wherein some tracks include diffusely reflecting regions. 19.A diffractive device according to claim 1 wherein some of the tracksinclude specularly reflecting regions.
 20. A diffractive deviceaccording to claim 1 wherein the variations in orientation, curvatureand spacing can be described by mathematical functions of two variablesin which the Hessian of second derivatives is non-vanishing except alongcertain characteristic lines within each track.